Haulers always desire to carry the maximum load possible. To prevent undue deterioration of highways, other road surfaces, bridges and the like, lawmakers have enacted legislation imposing weight restrictions on vehicles travelling over state and federal roadways. Typically, the weight restrictions are measured in terms of load per axle and the overall spacing between axles. It is readily apparent that if the number axles of the truck and the spacing thereof can be increased, a given vehicle may carry a heavier payload than would otherwise be permitted.
In order to increase the load capable of being carried by dump trucks, ready-mix cement trucks or other work type vehicles, trailing tag axle assemblies have been attached to the vehicles at the rear ends thereof to more sufficiently distribute the weight of the cargo thereon. Each trailing tag axle assembly has a trailing tag axle at the back of the assembly, with a set of non-powered wheels attached to the axle. The tag axle assemblies generally assist in balancing heavy loads and permit carriage of loads not otherwise permitted due to improved weight distribution provided thereby. As the tag axle is not required, and even prohibited from ground engagement and travel, during a "return" trip (i.e., empty or deadhead transport), retractability or selective stowage is a sought after feature for such tag axle assemblies.
An initial approach to tag axle retractability, meeting with limited success and having a narrow range of hauling applications, was that illustrated in U.S. Pat. No. 4,082,305 issuing to Allison et al. Here, the auxiliary wheels are not satisfactorily elevated when in a stowed condition to clear curbs, rough terrain, etc. as is typically encountered when leaving a finished road to enter a construction area.
Subsequent efforts addressing ground clearance issues focused upon elevatingly rotating tag axle assemblies at the end of arms, which may or may not be extendible, to a substantial, often times prohibited, height above the ground using hydraulic means operatively coupled between the vehicle and the arms. Such assemblies are typically stowed for deployment in an overhead and rearward position with respect to the hauling vehicle. As known assemblies are heavy, and particularly bulky, stowage becomes a challenge, necessitating a balance be struck between a variety of factors, for instance, available space for stowage, deployment clearance (i.e., the space over and above the vehicle, or portions thereof, in the vicinity of the stowage area), and the spacing of the tag axle relative to the other vehicle axles when deployed so as to achieve the desired additional or incremental load bearing capacity for the vehicle. Furthermore, in a stowed condition, the assemblies are frequently positioned substantially above the center of gravity for the vehicle, making the entire vehicle top heavy, and often times interfering with the loading and unloading operations associated with hauling.
Further disadvantages of heretofore known tag axle assemblies result from the inability of the trailing tag axle assembly to respond quickly, smoothly and efficiently to bumps and dips in the road while the axle is deployed for supplemental load bearing. This is a potential safety concern for the driver of such vehicles, and those drivers with whom he or she shares the roadways. Additionally, since load hauling vehicles are frequently driven over unpaved, rough and uneven surfaces, continuous jarring of the components of the tag axle assembly is a certainty, with twisting and a variety of other forces detrimentally acting thereon. Such repeated stress affects the structural integrity of tag axle assemblies, limiting its useful life and thereby implicating the cost of hauling such loads.